You'd need to either
1. Buy out your contract
2. Take out an additional contract
3. Buy it on pay as you go or sim free
O2 ran a scheme when the iPhone 4 came out where people could upgrade early by paying £20 per month remaining as an upgrade charge although no other network has ever run anything similar and a little bit of maths revealed this wasn't as good an offer as it seemed when you factored in the tariff downgrade policy at the time permitted people to eventually reach £10 per month on their tariff